The present invention relates to a heat transfer sheet and, more particularly, to a heat transfer sheet which is useful for heat transfer systems using sublimable (or thermally migrating) dyes and is so improved in storability and dye migration at the time of heat transfer that it can impart a much more improved density to the resulting image.
As replacements to generally available typographic and printing techniques, ink jet, heat transfer and other systems have been developed to provide excellent monochromatic or full-color images in a simple and fast manner. The most excellent of all is the so-called sublimation heat transfer system making use of a sublimable dye, which provides a full-color image having an improved gradation or gray scale and comparable to a color photograph.
In general, a heat transfer sheet used with the sublimation type of transfer system typically includes a substrate film such as a polyester film, which is provided on one side with a dye layer comprising a sublimable dye and a binder and on the other side with a heat-resistant layer for preventing it from sticking to a thermal head.
The dye layer of such a heat transfer sheet is overlaid on an imageable or image-receiving material having an image-receiving layer formed of polyester or other resin, and heat is applied to the heat transfer sheet from its back side in an imagewise manner, thereby causing the dye in the dye layer to migrate onto the image-receiving material to form the desired image.
According to such a heat transfer system as mentioned above, only the dye migrates from the dye layer into the image-receiving material, leaving the binder on the substrate film. In this case, the better the dye migration, the clearer and denser the resulting image. Thus, it is preferred to use as that binder a resin having a reduced or limited affinity for the dye used. From such a point-of-view, wide use has so far been made of cellulosic resins less dyeable with dyes.
A problem with forming the dye layer with the cellulosic resin, however, is that, as a result of the cellulosic resin having a low compatibility with the resin, the dye tends to precipitate out and crystallize during the storage of the heat transfer sheet, giving rise to a sharp drop of its sensitivity and thus failing to give an image of high density.